Ultrasound pulse echo methods of the kind described above are known from the text: "Werkstoffpruefung mit Ultraschall", Springerverlag, Fifth Edition, pp. 176-179, 349 and 382-389, by Josef Krautkraemer and Herbert Krautkraemer. It is customary for the known pulse echo methods to utilize transversal as well as longitudinal sound waves with respect to the shaft. Accordingly, for testing shafts, the test head delivering the ultrasound waves having a frequency range of 2 to 5 MHz, is placed adjacent the end face or cylinder face of the shaft and is coupled thereto. Additionally, ancillary wedges are arranged in front of the test head to allow an angular alignment of the test head's sound beam. The choice of the coupling point or the mounting surface for the test head depends upon the geometric nature as well as the material composition of the test piece.
It is known for shorter shafts, such as the axles for a set of railroad wheels, to sonicate the axle shaft from the end face. In the case of longer test pieces, such as turbine shafts, the test heads are guided along the cylinder surface usually with the aid of mechanical devices. To move the test heads along the cylinder surface presupposes that the cylinder surfaces are accessible. A problem arises for long shafts when the cylinder surface is not accessible. An example are wheel discs which have blades that are mounted on a turbine shaft and require shoulders on the shaft for fastening. The coupling of the wheels to the shaft can form starting points for cracks which arise in operation of the shafts. Nondestructive material testing using the pulse echo method along the cylinder surface requires a large and cumbersome number of technical devices as the turbine shaft must first be removed, at least from the stator or housing, for the test. Further, in order to perform a complete test, the wheel discs must be removed as most cracks are located under the wheel discs.
Thus, there is a need to provide a method for detecting cracks in a shaft by means of ultrasound pulse echo methods which permits detecting cracks from the end face of long shafts while the shaft remains in its assembled state and which is reliably reproducible and effected in a simple manner. Such a method should also permit reliable detection of cracks or incipient cracks existing at the shaft circumference. There is also a need for a mechanical device suitable for carrying out such a method.